In electrostatic printing and copying systems, an electrostatic writing head is commonly used to deposit a latent image of electrical charges onto a recording medium such as a paper web. The paper web carrying the latent image is then directed to a liquid toning applicator which deposits charged toner particles onto the paper web, thereby developing the latent image.
Such liquid toning applicators can be classified as pressure-type or suction-type. In suction-type applicators, a liquid pump is used to draw toner from a toner reservoir into the applicator and across one or more channels in the face of the applicator, wherein the toner comes into contact with the paper web. The flexible web series to seal the face of the applicator thus permitting the pump, connected to the applicator outlet, to create a vacuum or suction in the channels and thereby draw toner into the applicator from a toner reservoir. The outlet of the toner pump returns the spent toner to the reservoir. In this manner a continuous recirculation of the toner occurs. The flexible paper web thus forms a liquid seal around the periphery of the applicator assuring that pressure differences are maintained. The face of the applicator must be oriented upward to insure that the toner is not spilled. If the paper web is pulled away from the applicator, damaged, or is simply depleted, the vacuum seal is broken, all liquid pumping ceases, and the toner flows gravitationally back into the toner reservoir. As a result, suction-type toner applicators are virtually leak-proof.
Despite the advantage of leak protection, suction-type applicators are severely limited by slow toning speeds. Since the paper web comes into contact with the toner in a channel of decreased pressure, the edges of the channel must support the web against the inward force. For this reason, the channels must be no greater than about a tenth of an inch wide in order to prevent the paper web from being pulled down into the toning channel and cutting off the flow of toner. To compensate for this narrow channel size, several parallel channels are incorporated into the face of the applicator. A practical design has these narrow channels aligned perpendicular to the direction of movement of the web. Such toning applicator systems are referred to as "cross-flow" applicators.
As the speed of the web passing over a cross-flow applicator is increased, more and more channels are required in order to insure that adequate toning occurs. As additional channels are incorporated into the applicator, the paper web is subjected to greater sliding friction. Furthermore, narrower channels, which can typically be up to 36 inches long, require stronger suction from the liquid pump in order to get the toner to quickly flow therethrough. This adds to the downward force on the paper web, and further increases the sliding friction. As web speeds approach 2 inches per second, the speed necessary for high speed plotting or printing, the number of required channels is so great that the sliding friction of the web against the applicator becomes impractical. Thus, suction-type applicators are restricted optimally to web speeds below approximately 1 inch per second.
Pressure-type applicators function in a slightly different manner. Instead of using a vacuum to draw toner across the face of the applicator, the toner is pumped out of a toner reservoir using positive pressure. In one embodiment of a pressure-type applicator the toner is pushed by positive pressure through a channel in the applicator and finally received by an inlet port on the opposite side of the applicator channel for continuous recirculation. In order to compensate for the positive pressure exerted on the paper web by the pressurized toner, a backing surface is placed behind the web to hold it against the applicator. Thus, pressure-type applicators do not create suction in the applicator channels, and sliding friction of the paper web is greatly reduced.
Since suction is no longer a problem, pressure-type applicators are freed from the "narrow-channel" geometries of suction-type applicators. For instance, the channel can be almost the full size of the applicator's contacting surface, extending across the entire width of the paper web, and the full extent of the applicator in the direction of movement of the web. One such "full-width" channel can be as effective as ten or twenty cross-flow channels, and almost totally without paper sliding friction. As a result, far higher web speeds can be employed without system compromises or image quality sacrifices. Furthermore, the flow of the toner may be parallel to the paper web direction, i.e. the toner in the channel may flow opposite or in the same direction as the paper movement. Such a short but wide channel requires reduced pressures and liquid velocities, both of which characteristics benefit the toning process.
In another embodiment of the pressure-type applicator, a rotating "toning""roller in contact with the web is either wetted by partial emersion or else sprayed with liquid toner. In either case a bucket or funnel must be used to contain the toner liquid and prevent leakage. By proper design the toning roller can be used as well for drying the image after toning. Such a roller system works well at low web speeds but is not suitable for high web speeds. The toner containing bucket also presents great difficulties if used for a single-applicator system for multi-color toning. This is discussed below.
Although pressure-type applicators offer advantages such as reduced friction and increased channel size, they are hampered by toner leakage. Even when precisely designed and constructed, toner leakage still occurs around the edges of the applicator. Often there is no attempt to minimize leakage, and the flow of toner is simply collected by a bucket or funnel situated underneath the applicator, and then recirculated. Using buckets increases the cost of the system, and the buckets also require occasional cleaning. Furthermore, in applicator systems in which a single applicator is used for several different colored toners, such buckets result in cross color contamination due to toner mixing in the buckets.
Therefore it is an object of this invention to provide a toner applicator having the speed advantages of a pressure-type applicator, without toner leakage, and which is suitable for applying multi-colored toners from a single applicator.
A further object of this invention is to provide such a toning applicator with built-in drying means in the form of an air knife for removing excess liquid from the paper web.